The kinetics of migration of 125I-labeled immunoglobulins and antibodies and 125IUDR labeled lymphocytes from blood to cerebrospinal fluid and to brain and the extent of their sojourn there will be followed in hamsters under three different experimental conditions: 1) normal hamsters, 2) hamsters treated with chemicals that cause reversible blood brain barrier damage, and 3) hamsters with measles infection of the central nervous system (CNS) or with a latent infection. The chemicals used in treatment of the hamsters will be urea and a derivative of piperidine, 1-benzyl-4-dimethyl amino piperidine. Hypermolaric concentrations of urea cause opening of tight junctions of the blood brain barrier and the piparidine derivative causes hydropic degeneration of the choroid plexus, a vital tissue of the blood brain barrier. Both changes are reversible and leave the rest of the brain unaffected. At the ultrastructural level the path of migration of 125I labeled Igs and 125IUDR labeled lymphocytes across the choroid plexus will be followed by autoradiography. The presence of Fc receptors on the epithelia of the choroid plexus and their possible role in the passage of selected Igs will be investigated. The basic information obtained from these studies on the route of passage and kinetics of ingress from blood to CNS of antibodies and antibody-forming cells and their length of stay in the CNS as functioning elements should aid in an understanding of their presence in the CNS of patients with multiple sclerosis and other CNS disease of obscure origin.